Create a curved face between two bezier curves in Blender 2.8
It looks like each side is a separate mesh object (You have two white 'active' vertices, one for each mesh). If you select both in object mode you can join them into one using Ctrl-J. 

Editing multiple meshes at the same time is a new feature in 2.8, and allows for many useful operations, but if they are separate objects, they still can't share any of their actual geometery.